Question

how does a resonance stabilise a molecule?

Answer 1

In resonance normal state of a molecule is represented not by a single valence-bond structure but by a combination of several alternative distinct structures. The molecule as a result resonates among the several valence-bond structures or to have a structure that is a resonance hybrid of these structures. The energy calculated for a resonance hybrid is lower than the energies of any of the alternative structures; the molecule therefore is then said to be stabilized as a result of resonance